But too often old philosophy is taught and read in a vacuum, not as history but as a source of practical modern philosophy. This results in repeated resurgences of bad ideas that were refined away by more modern advancements in philosophy, but are revived when people read old philosophy uncritically. Few people have an understanding of the refinements to even refute many older philosophers claims.
Moreover, there are many accepted ideas which we take for granted that should be more deeply questioned which come from philosophy. For example, the pervasive Socratic idea that mind and body are separate entities is all but disproven by modern understanding of the nervous system, but it's deeply ingrained in our culture, and needs less reinforcement, not more.